She can file Form I-539 for Extension of Status at any point up until her status expires. The application just has to be officially received by USCIS before her status expires.
If she filed her Extension of Status application before her status expired, she can stay in the US while it is pending even if her status expires in the meantime. The government will generally not deport her for being out of status while her Extension of Status is pending. In addition, she does not accrue "unlawful presence" while her timely-filed, non-frivolous Extension of Status application is pending, even if her I-94 is expired. If her Extension of Status is approved, she will be retroactively granted status from when her last status expired.
See this brochure:
What if I file for an extension of stay on time but USCIS doesn’t make
a decision before my I–94 expires?
Your lawful nonimmigrant status ends, and you are out of status, when
your Form I-94 expires, even if you have timely applied to extend your
nonimmigrant status. Generally, as a matter of discretion, USCIS will
defer any removal proceedings until after the petition is adjudicated
and USCIS decides your request for extension of nonimmigrant status.
Even though you are not actually in a lawful nonimmigrant status, you
do not accrue “unlawful presence” for purposes of inadmissibility
under section 212(a)(9)(B) of the Immigration and Nationality Act,
while your extension of status application is pending if it was filed
prior to the expiration of your Form I-94.
If your application for an extension of stay is approved, the approval
will relate back to the date your Form I-94 expired, and your status
while your application is pending will then be considered to have been
If your application is denied, you may be required to cease employment
and depart the United States immediately.
In addition, any nonimmigrant visa in your passport granted in
connection with your classification becomes void. Once your visa is
void, you must submit any new visa application at a U.S. consulate in
your home country (not a third country, except in rare instances as
determined by the U.S. Department of State).
If her timely-filed, non-frivolous Extension of Status application is denied, and her I-94 has expired, and she is still in the US on the date of the denial, she will start to accrue "unlawful presence" from the date of the denial.
See AFM chapter 40.9.2(b)(3)(D)(iv):
If a timely filed, non-frivolous request for EOS or COS is denied for
cause, unlawful presence begins to accrue the day after the request is
And 9 FAM 302.11-3(B)(1)(b)(4)(b)(ii):
b. DHS has interpreted "period of stay authorized by the Secretary of
Homeland Security," as used in this context, to include:
(4) For aliens who have applied for extension of stay or change of
nonimmigrant classification and who have remained in the United States
after expiration of the From I-94 while awaiting DHS's decision, the
entire period of the pendency of the application, provided that:
(a) the alien does not work unlawfully while the application is
pending and did not work unlawfully prior to filing the application;
(b) the alien did not otherwise fail to maintain his or her status
prior to the filing of the application (unless the application is
approved at the discretion of USCIS and the failure to maintain status
is solely a result of the expiration of the Form I-94), and further
(i) [...]; or
(ii) if the application was denied or the alien departed while the
application was still pending, that the application was timely filed
"Unlawful presence" doesn't cause a ban unless she leaves the US after accrues more than 180 days of "unlawful presence". So she shouldn't have to worry about a ban as long as she leaves within a reasonable amount of time after she learns about the denial.
The other consequence of her being still in the US when her Extension of Status is denied if her I-94 is expired, is that INA 222(g) is triggered, which means the visa she used to enter the US is automatically voided, and she would have to apply for US visas from her country of nationality from then on.
Also, USCIS has a policy where they will issue Notices to Appear (NTAs) to people after an I-539 denial if they are unlawfully present after the denial, although I am not sure whether they are still doing that during the COVID-19 outbreak. According to this teleconference, they will wait for a 33-day period (the period to file a Motion to Reopen or Reconsider) after the denial before they issue the NTA, so this basically means your mother should try to leave within 33 days of a denial.
On the other hand, if she leaves the US while her timely-filed, non-frivolous Extension of Status application is still pending (i.e. before it is approved or denied), she does not accrue any "unlawful presence", and her visa is not voided.
See AFM chapter 40.9.2(b)(3)(C):
Departure from the United States while a request for EOS or COS is
pending, does not subject an alien to the 3-year, 10-year, or
permanent bar, if he or she departs after the expiration of Form I-94,
Arrival/Departure Record unless the application was frivolous,
untimely, or the individual had worked without authorization.
And the 9 FAM 302.11-3(B)(1)(b)(4)(b)(ii) passage quoted previously.
And it does not void her visa under INA 222(g), according to 9 FAM 302.1-9(B)(1)(c)(4)(B):
[...] In addition, if an alien departs after the date on the Form I-94
passes, but before his or her application for extension or change of
status has been decided by USCIS, they must be subject to a blanket
exemption from INA 222(g), if the application was filed in a "timely
manner" and is "nonfrivolous" in nature. You may consider an
application nonfrivolous if it is not, on its face, a groundless
excuse for the applicant to remain in the United States to engage in
activities incompatible with his or her status. Posts may be
satisfied that an alien filed in a timely manner using evidence such
as a record in USCIS Person Centric Query Service (PCQS) or the dated
receipt or canceled check from USCIS for the payment of the
application fee to extend or change status together with evidence of
the expiration of the alien's legal status.
The meaning of being timely-filed and non-frivolous are described in AFM chapter 40.9.2(b)(3)(C):
The requirement that the application was timely may be established
through the submission of evidence of the date the previously
authorized stay expired, together with a copy of a dated filing
receipt, a canceled check payable to USCIS for the EOS or COS
application, or other credible evidence of a timely filing.
To be considered non-frivolous, the application must have an arguable
basis in law and fact, and must not have been filed for an improper
purpose (such as to prolong one’s stay to pursue activities
inconsistent with one’s status).
In determining whether an EOS or COS application was non-frivolous,
DOS has instructed consular posts that it is not necessary to make a
determination that USCIS would have ultimately ruled in favor of the
And 9 FAM 302.11-3(B)(5)(b-c):
b. [...] Therefore, for all cases involving potential inadmissibility
under INA 212(a)(9)(B) whether under the three-year bar of
212(a)(9)(B)(i)(I) or the 10-year bar of INA 212(a)(9)(B)(i)(II), DHS
has decided to consider all time during which an application for
extension of stay (EOS) or change of nonimmigrant status (COS) is
pending to be a period of stay authorized by the Secretary of Homeland
(1) The application was filed in a timely manner; i.e., before the
expiration date of the Form I-94, Arrival and Departure Record;
(2) The application was "nonfrivolous"; and
(3) The alien has not engaged in unauthorized employment (whether
before or after April 1, 1997).
c. To be considered "nonfrivolous" the consular officer must find that
the application had an arguable basis in law and fact and must not
have been filed for an improper purpose (e.g., as a groundless excuse
for the applicant to remain in activities incompatible with his or her
status). It is not necessary to determine that the DHS would have
approved the application for it to be considered nonfrivolous.
So the question is whether her application of Extension of Status is "frivolous" or not. Note that it is possible for an application to be "non-frivolous" even if USCIS is likely to deny it; it just needs to have an "arguable basis in law and fact" and not be "filed for an improper purpose".
In my (non-professional) opinion, an application of Extension of Status for the reason of not being able to return to her home country to due COVID-19 flight bans is not frivolous. In the USCIS page on Special Situations, it mentions that for applications of Extension of Status and Change of Status, USCIS will take into consideration how special situations prevented your departure.